Populations and speciation Flashcards
What does the hardy-weinberg principal predict?
- the proportion of dominant and recessive alleles in a population remain constant from one generation to the next
What are the 5 conditions assumed for the hardy-weinberg principle to hold true?
- no mutations
- the population is isolated - there is no flow of alleles into or out of the population
- all alleles are equally likely to be passed on to the next generation
- large breeding population
- random mating occurs
What are the formulae for hardy-weinberg?
p + q = 1
where p = frequency of dominant allele and q = frequency of recessive allele
p^2 + 2pq + q^2 = 1
where p^2 = homozygous dominant individuals, q^2 = homozygous recessive individuals and 2pq = heterozygous individuals
What is the gene pool?
all of the alleles of all genes of all the individuals in a population at a given time
What is allele frequency?
the number of times an allele occurs within the gene pool
What can genetic variation arise as a result of?
- mutations
- meiosis - independent segregation, random assortment, crossing over
- random fertilisation of gametes
How can variation arise as a result of environmental factors?
- seeds may have genes that allow them to grow very tall
- if they germinate in an environment with poor light and lack of nutrients, it will be short
- therefore variation is often due to a mix between genetic and environmental influences
What is the role of over-production of offspring in natural selection?
- when there are too many offspring for the available resources, there is intraspecific competition
- the greater the numbers, the greater the competition and the more individuals die
- therefore those best suited to the conditions will survive and pass on their advantageous alleles, increasing the frequency of alleles within the population
What is disruptive selection?
- favours extreme phenotypes at the expense of intermediate phenotypes, and is the most important in bringing about evolutionary change
- eg. in salmon larger and smaller fish have an advantage over intermediate fish, because small males can sneak up on females in spawning grounds and hide from predators, whilst larger males are fierce competitors, therefore the number of intermediate fish will decrease
What is speciation?
the evolution of new species from existing ones due to reproductive separation
How does speciation occur?
- populations will become separated and undergo different mutations causing them to become genetically different
- natural selection then leads to changes in allele frequencies as the population adapts to the local environment
- they are then unable to interbreed to produce fertile offspring, and each population becomes its own species with its own gene pool
What is genetic drift?
- the gradual change in allele frequency of a population
- takes place in small populations because there is only a small variety of alleles and less genetic diversity
- as the individuals breed, the alleles passed on will quickly affect the whole population as their frequency is high
- this makes the population change rapidly making it more likely to develop into a separate species
What are the 2 forms of speciation?
allopatric and sympatric
What is allopatric speciation?
- the form of speciation where 2 populations become geographically separated so there is no gene flow eg. due to a river which prevents them from interbreeding
- if environmental conditions vary on either side, then natural selection will influence the populations differently, and each will evolve and adapt to their local conditions
- this leads to reproductive separation and the formation of a new species
What is sympatric speciation?
- speciation that results within a population in the same area causing them to become reproductively separated, as no genes flow between the 2 groups
What may be the causes of isolation in sympatric speciation?
- ecological - live in different environments in the same area
- temporal - reproduce at different times so do not exhibit courtship behaviours at the same time and are not fertile
- behavioural - different courtship behaviour
- mechanical - mismatch in reproductive parts
Which 2 reproductive isolation methods occur after mating has occurred?
- gametic - gamete incompatibility eg. sperm does not produce the correct enzymes to penetrate the egg
- hybrid sterility - physical inability to produce gametes and therefore any offspring
